"Set aside politics: every American should condemn such interference in the Bureau's work," Snowden wrote.

"This FBI director has sought for years to jail me on account of my political activities. If I can oppose his firing, so can you."

Democratic Senator Bob Casey called the firing of Comey "Nixonian".

He said: "Deputy Attorney General Rod Rosenstein must immediately appoint a special counsel to continue the Trump/Russia investigation.

"On March 20 Director Comey said, 'I have been authorised by the Department of Justice to confirm that the FBI, as part of our counterintelligence mission, is investigating the Russian government's efforts to interfere in the 2016 presidential election and that includes investigating the nature of any links between individuals associated with the Trump campaign and the Russian government and whether there was any coordination between the campaign and Russia's efforts.'

"This investigation must be independent and thorough in order to uphold our nation's system of justice."

The Democrats said that Trump had fired Comey due to his role in the FBI inquiry looking at if members of the Trump campaign colluded with the Kremlin.

Senate Minority Leader Chuck Schumer said: "Were these investigations getting too close to home for the president? This does not seem to be a coincidence."

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But Trump himself hit back with a Tweet that had a touch of venom.

Comey's deputy, Andrew McCabe, will take over as the search for a replacement has started. Reuters reported how FBI staff are scheduling an emergency high-level meeting amid speculation about who would replace Comey in the top job.